Oxygen air is decomposed in converting it with hydrogen to water: there is no oxygen or hydrogen air in water; their _elements_ are the constituents of water. Oxygen is decomposed by respiration; when inspired, it is not expired, but nitrogen, which must have been one of its constituents, and from there being nothing to constitute the expiration but the previous inspiration the proposition is proved. The constituents of oxygen are--nitrogen, _a highly rare imponderable element_ and medium of space. The first is the most ponderable element of nitrogen air; its atoms are the largest of all others of the elements of matter, and, it may be said, they constitute the substance of the framework of all ponderable or gross formations. Davy says, "the properties of nitrogen are altogether negative;" the same applies to every other kind of air, all being constituted of _inert_ atomic substance, consequently of inactive essence; and all being alike in every respect but in the size of their atoms. The imponderable element being highly evanescent, is never found alone, and is always connected with nitrogen; hence simple nitrogen is obtainable only from bodies, or by deoxygenating atmospheric air. Atmospheric air is nitrogen, plus the imponderable element; and when the nitrogen is saturated with the same element, the air is oxygen: hence, whichever is inspired, nitrogen is expired. From nitrogen being evolved copiously from water in vacuo, and from ice being convertible to nitrogen, according to Priestley, so is nitrogen a constituent of water, also of the gases into which water is decomposable; but as it cannot belong to the hydrogen, owing to its superior levity, it must to the oxygen; which is confirmation of the above, that nitrogen is a constituent of oxygen air or gas. THE USE OF OXYGEN IN PROMOTING COMBUSTION.